The invention is based on a device for venting a fuel tank and on a process for checking the functional capability of the device as generically defined hereinafter. German Offenlegungsschrift 40 03 751 discloses a venting device in which the fuel vapors evaporating in a fuel tank are removed by suction so that for environmental protection reasons emissions, particularly of hydrocarbons, into the environment can be averted. From the fuel tank, the fuel vapors are introduced via a tank line into an adsorption filter embodied in the form of an activated charcoal filter, which adsorbs the fuel vapor or retains and temporarily stores it. Because an adsorption filter of this kind has a limited holding capacity, it must be regenerated, that is, rinsed with air, for which purpose a ventilation line is provided at the adsorption filter. When an internal combustion engine is in operation, a vacuum prevails in an intake pipe of the engine, so that air flows from the environment via the ventilation line into the adsorption filter in order to entrain the fuel stored in the adsorption filter or to mix the fuel with the aspirated air so that it can be introduced in the form of fuel vapor via a regenerating valve into the intake pipe in order then to be burned in a combustion chamber of the engine. The introduction of the fuel vapors, though, must be done deliberately, that is, only in selected operating ranges of the engine, so that on the one hand the exhaust emissions of the engine are not increased, and on the other hand, highly smooth engine operation remains guaranteed. To achieve this, the regenerating valve inserted between the adsorption filter and the intake pipe is triggered by an electronic control unit so that it opens only intermittently, in selected operating ranges of the engine, which are dependent on the vacuum in the intake pipe. To monitor the combustion of the engine and the opening times of the regenerating valve, a lambda control is typically provided in combination with a so-called adaptive mixture control, which constantly monitors the exhaust emissions by means of a lambda sensor placed in the exhaust flow of the engine, in order, for example by altering the injection times of the injection units provided in the engine to adjust the mixture composition at a predetermined air-fuel quantity ratio, at which ratio the combustion proceeds almost completely, with only slight exhaust emissions.
In venting devices of the prior art, the individual connecting lines from the fuel tank to the intake pipe are embodied as single walled, for example in the form of simple hoses. Fuel vapors can escape into the environment through the tiniest holes in the hoses, which occur from aging, without the driver of a vehicle equipped with such a venting device being able to notice it.
That is why legislators, particularly in California, have stipulated regular monitoring of the functional capability and tightness of venting devices of this kind. In this checking process, a stop valve provided at the adsorption filter only for checking purposes is for example manually closed, so that no air can flow into the adsorption filter via the ventilation line, after which the regenerating valve is brought to an open position. After the engine is started, the increase of the vacuum is measured by a pressure sensor of the engine installed in the fuel tank, which increase must be maintained for ample time in the fuel tank after the closing of the regenerating valve, so that leaks from the venting device can be precluded with high probability.
This checking process is relatively complicated, since to monitor the vacuum in the fuel tank, the regenerating valve and the stop valve must be brought to a checking, or open, position, and after the checking process must be returned to a functional position. It can happen that the stop valve at the adsorption filter remains in the closed position, for example because of a defect or carelessness. If the engine, with the regenerating valve closed, is then for example exposed to a strong temperature rise, an excess of pressure can build up in the fuel tank due to the increasingly evaporating fuel vapors; for safety reasons, at a predetermined excess pressure, this excess pressure must then escape into the environment via a safety valve provided on the fuel tank, in the course of which fuel vapors escape into the environment as well.